This invention relates to rotors and pressure screens and, more particularly, to a rotor for a screen for removing contaminants from a suspension of paper making pulp.
In the manufacture and treatment of papermaking pulp, pressure screens are used to separate and remove undesirable contaminants from the process. These contaminants may take the form of foreign materials introduced into the process with the raw material, or they may be remnants of the pulp production process itself, such as fiber bundles (also called “shives”) left over from the production of chemical pulp, or undefibered flakes that were not reduced to good fiber in a pulper.
Separation of this undesirable material is referred to as screening, and requires passing the pulp slurry through very small openings; most typically slotted screen cylinders are used with slot openings of between 0.10 and 0.40 mm.
Screens work with pulp in slurry form at an oven-dry consistency of about 4-5% or less, most commonly in the range of 2-3%. These machines have a continuous liquid reject stream that must be further treated to recover good fiber; therefore, multi-stage cascaded systems are usually installed.
Closed pressure screens in which a flat or cylindrical screen is used to separate a suspension of paper-making pulp into an accepts pulp fraction and a reject fraction have long been used for paper pulp cleaning. Such pressure screens commonly employ a generally cylindrical foraminous screening member, which may have an aperture pattern made up of either holes or slots. A rotating impulse member is positioned to operate adjacent a surface of the screen, which is commonly, but not always, an inner inlet surface, to maintain the stock suspension in a state of agitation and to provide pressure impulses to aid the screening function. The rotating-member may comprise a drum-type rotor in which protuberances or foil-shaped sections are mounted on the outer surface and move adjacent to a screen surface, or foils may be mounted on generally radially extending arms for rotation adjacent the screen surface.
Commonly, the pulp stock suspension to be screened is brought in at or adjacent an axial end of a cylindrical screen and, during screening, moves axially between the inlet surface, as stated above, commonly the inner surface of the screen cylinder, and the surface of the aforesaid drum-type rotor. At the same time, a rejects fraction is created by the inhibition or screening out of undesirable material which does not pass through the apertures or openings in the screen cylinder, and this undesirable material also moves axially along the screen surface until it reaches the end of the screen axially opposite the inlet end, where it is directed to a rejects accumulation chamber and then to a rejects outlet.
Conventionally, the stock suspension enters at one end of the screen, or enters at the center of the screen and flows in opposite directions over the screen. The multiple foils perform the well-known impulse and screening function such that the fibers are accepted through the perforated or slotted screen while the larger or longer material that is unable to go through such perforations is retained within the screening zone until it reaches the rejects outlet.
It is also known that a pressure screen can be a single screen or a plurality of separate screens, divided into a plurality of axially spaced screening bands or zones, with means provided for applying the stock suspension under pressure directly to the inlet side of the screening surface, at each zone. Such axially disposed zones individually form a portion of the total axial extent of the screening means. At least one rejects receiving or collection area is provided for each such zone.
In current screens, some important features are that the separation barrier (screen cylinder) has very small openings, usually slots of 0.15 to 0.30 m in width. As the flow travels toward the other end (in this example, the bottom), good fibers in the liquid slurry pass outward through the screen plate openings, while contaminants (especially shives) continue until they pass out of the reject end of the screening zone.
As screens become larger, the area of the screening surface increases roughly with the square of the diameter (assuming the proportions of diameter to height are held constant). The entry area into the screening space, which is the annulus between the drum-style rotor and the screen cylinder, however, only increases roughly linearly with diameter. This means that as the screen gets larger, the entry velocity increases if the same flow per unit of screen plate area is to be maintained.
At some point, this increased velocity will cause one or both of an unacceptably high pressure drop, or performance degradation of the machine, because the desired flow velocity relationship between the pulse-generating elements on the rotor and the fluid is destroyed.
The problem thus to be overcome is that the entry velocity into the annular space between the drum-style rotor and the screen cylinder gets extremely high as screens become larger (the screen plate area goes up as the square of diameter, but the opening area goes up linearly).
At least two conventional offerings have sought to overcome this problem, using fundamentally the same approach. They reduce the height of the screen cylinder relative to the diameter, which increases the ratio of the inlet area to the screening area.
This approach has significant disadvantages. It either makes the rotor very complicated to manufacture (see FIG. 4) or it makes the machine very expensive to build (see FIG. 5).
The FIG. 4 design 10 uses the concept of stacking two short screens one on top of the other, with the inlet-to-reject flow direction being the same in both. Each part is only half the height of the cylinder; therefore the total entry area into the annular chamber effectively becomes twice the size.
This is executed by having a normal entry at the top. Just above the halfway point down the screen cylinder surface, scoops 11 on the rotor facing in the forward direction draw the flow inward into a channel inside the design, and from there it goes down to the bottom and out the rejects outlet.
At the top of the rotor 10 there is also an annular chamber open at the top slightly closer to the centerline than the normal entry. Some pulp (ideally one-half) passes downward in this chamber. It exits the rotor radially outward through a circumferential slot 12 located just below the halfway point, and just below the scoops 11 that picked up the flow from the top half. The second part of the flow now travels downward as it would in a conventional screen and out into the reject outlet at the bottom.
The FIG. 5 design 15 combines two screens, one on top of the other, but does it as if they were totally separate screens. The inlet is in the middle, with separate rejects at the top and bottom. In this case, the inlet-to-reject flow directions are opposite to each other.
One disadvantage to this approach is cost. Many more components and connections are required than would be necessary if it were a single, uncomplicated design. Another is that it is more complicated to disassemble for maintenance than a more conventional construction would be.